The present invention relates to the quantity determination of a component present in a small amount in a gaseous mixture by absorption of a radiation which is indicative of said component. The term "gaseous" must be interpreted in the widest sense as designating not only a gas properly speaking but also a vapor or a dispersion.
Numerous processes are already known for determining the quantity of a component present in a small amount in a gaseous mixture. Processes in particular are known of the type in which a radiation beam having passed through the mixture and whose spectral composition contains at least one spectral absorption line characteristic of the component alternately traverses two channels one of which comprises a given amount of the gaseous mixture and the other of which serves as reference.
It is an object of the invention to provide an improved process and device of that type. It is a more specific object to provide a device which is simple in construction and however of high sensitivity. It is still another object to provide a simple solution to the problem of selectivity of the measurement in the case where absorption spectral lines belonging to parasite gases are superimposed on those of the gaseous component whose amount has to be determined. Finally, in a particular embodiment, it is an object of the invention to eliminate the influence of a possible polarization of the light beam which is passed through the mixture.
To this end, a process of the above-defined kind according to the invention includes the step of constituting the second channel over which the radiation beam travels as a diaphragm having a radiation transmission substantially equal to that of the first channel in the absence of said component in the mixture. The output beam from the two channels is fed to a detector detecting the intensity variations of the beam, through a filter selecting said characteristic radiation.
The invention proposes also a device for the quantity determination of a component present in a small amount in a gaseous mixture, comrising means for directing alternately a radiation beam containing at least one spectral absorption line which is characteristic of the component, along a first channel which comprises a volume of said component and a second channel comprising a diaphragm calibrated so as to have a transmission substantially equal to that of the first channel when the mixture does not contain said component, as well as means for directing the beams coming from the two channels, through a filter transmitting a given fraction of the spectral absorption band of the component to be detected, towards a detector associated with means for measuring the variations of the output signal of the detector.
To increase selectivity, when the mixture is likely to contain a small amount of parasite gas, a given amount of the parasite gas, greater than that likely to be passed through by the beam in the second channel, may be interposed in the optical path common to the two channels.
The incident radiation beam may be formed by natural light. A light source will very often be used adapted to the spectral range of the characteristic radiation, which may, depending on the case, be in the ultra-violet, the visible or the infrared, (this latter case excluding the use of natural light). In the case of measuring the concentration of gas in the atmosphere, the above-defined elements may be combined together in the same apparatus, but the emission of the source through the atmospheric zone where the measurement takes place is then reflected by a directive reflector placed at a distance, for example by a catadiopter which removes the problems of aiming the reflector.
The invention will be better understood from reading the following description of particular embodiments given by way of non limiting examples.